Rendering a pixelated image
Found a solution in compositing based on this BlenderArtists thread.

To pixelate a render layer or image, you simply need to use the distort scale node to scale it down, use the pixelate filter node, then scale it back up.

The amount of scaling up and down control the amount of pixelation.



EDITED: If you really want to reproduce the effect from the original image, with the pixelated outline as well, you need a slightly more complicated node setup:



Download blend file here.

Original vs. pixelated+outline side by side: